Described below is a danger detector, in particular a smoke detector, for operation in an area having an increased radioactive radiation disposition, with the danger detector having at least one detector unit for detecting at least one danger characteristic, a semiconductor component and other electrical components, at least for outputting an alarm signal.
Furthermore, the danger detector may be embodied as a linear smoke detector, having an opto-transmitter for emitting a light beam which traverses a measured distance, and having an opto-receiver for receiving the emitted light beam at the end of the measured distance.
The danger detector involves a smoke detector for example, such as an optical smoke detector or a thermal detector. Optical smoke detectors can be based on the scattered light principle or on the opto-acoustic principle, for example. If the danger detector is a thermal detector, then the temperature currently present in the surroundings of the thermal detector is detected, for instance by a temperature-dependent resistor. The danger detectors under consideration can also be flue gas detectors, which have a gas sensor as the detector unit, such as a gas FET (Field Effect Transistor) for example.
Furthermore, the danger detectors can involve motion detectors which have a PIR (passive infrared) detector unit for motion detection. The danger detectors under consideration can also have combinations of the afore-mentioned detector units.
Linear smoke detectors are based on the extinction principle. They are employed in particular in large or narrow spaces, for example in corridors, warehouses, factory buildings and in airplane hangars and are mounted below the ceiling on the walls. In a first embodiment, transmitter and receiver are located opposite each other and no reflector is required. In a second embodiment, the light beam emitted by the transmitter is deflected via a reflector back to the receiver. Transmitter and receiver adjoin each other. The measured distance of such types of linear smoke detectors is typically in the range from 20 m to 200 m, which in the case of the first embodiment corresponds to the equivalent distance between transmitter and receiver. In the second embodiment, the distance between transmitter/receiver and the reflector corresponds to half of the measured distance.
The area with increased, in particular with high radioactive radiation disposition, can be for example a nuclear area or in space. Nuclear areas are in particular spatially delimited areas within a nuclear power station, a nuclear reprocessing plant or a final or intermediate storage facility for radioactive waste, for example.
Radioactive radiation means ionizing particle or electromagnetic radiation which comes from radioactive materials and is able to tear off electrons from atoms and molecules, so that positively charged ions or molecule residues remain. Whereas alpha and beta radiation as particle radiation can even be screened by materials having a thickness of a few millimeters, effective screening against electromagnetic gamma radiation is only possible with a large amount of material. Depending on the screening requirement, lead shields with shield thicknesses of a meter or more can be required.
Radioactive radiation generally has a destructive effect on electronic components, particularly on semiconductor components. Such components have very fine semiconductor structure patterns of less than 1 μm, in particular less than 100 nm. In this case all types of high-energy, ionizing radiation interact with a semiconductor crystal. Even if screening against alpha and beta radiation is comparatively simple to achieve, for instance by a sheet metal enclosure or a plastic enclosure for example, then the action of the gamma radiation on the screening or on the housing of the semiconductor components results to some extent in secondary alpha and beta particles, which in turn interact with the semiconductor crystal. Due to the interaction of such an irradiated particle with a lattice atom, the latter can be released from the lattice structure and this produces a vacancy. If it has sufficient transferred impact energy, the free atom can knock out further atoms, or migrate to an intermediate lattice position. This forms a so-called intermediate lattice atom vacancy complex.
An important effect of the interacting radiation is the production of crystal defects which generate additional energy states within the forbidden band along with recombination centers. These effects are accelerated in highly complex semiconductor microstructures, such as ASICs or microcontrollers, for example. On the other hand, resistors or capacitors are very rarely affected.
For this reason, rugged, discrete semiconductor components such as transistors or diodes may be used to take into account accelerated degeneration of the electrical parameters in the circuit, especially as predominantly radiation-hardened, older integrated semiconductor components, such as ICs, logic gates, etc., which have a pattern size of more than 1 μm and due to the advances in miniaturization are in short supply in the semiconductor market.
Due to the use of discrete semiconductor components, a minimum service life, for example 3 years, which meets the relevant requirements, such as those of a nuclear power station, for example, can therefore be realized. Such a requirement can be, for example, that a smoke detector has to withstand a radiation exposure or an energy dose of 0.25 Gy over a period of 3 years. Here the term Gy (Gray) is the SI unit of the absorbed energy dose D. In this case the energy dose absorbed with respect to time is termed the dose rate.
In an unrelated field, a semiconductor laser for applications in space is known from Chinese Patent Application CN 101841125 A. To an undescribed extent and by undescribed means, the temperature of the semiconductor laser is increased in order to accelerate annealing of damage caused by radiation, typically by protons and electrons.
A detailed description of the effect of radioactive radiation on electronic semiconductors, in particular the associated accumulated or temporary damage over time of such semiconductor components, is described in the dissertation “Component degradation due to radioactive radiation and its consequences for the design of radiation-hardened electronic circuits” by Detlef Brumbi, Faculty of Electronic Engineering at Ruhr University, Bochum, 1990.
A mathematical model for a stress method for MOS semiconductors at a high radiation rate, which enables precise long-term forecasts to be made concerning the rate of formation of induced holes (positive charges) in the semiconductor material due to the effect of radioactive radiation, in particular in the semiconductor oxide employed as an electrical circuit insulator, such as SiO2, for example, is proposed in the publication: IEEE Transactions on Nuclear Science, VOL. 37, NO. 6, DECEMBER 1990, titled “Modeling the anneal of radiation-induced trapped holes in a varying thermal environment”, by P. J. McWhorter, S. L. Miller and W. M. Miller of Sandia National Laboratories, Albuquerque, N. Mex., USA, December 1990. Based on this determined rate of formation, reliable data are obtained on the long-term failure modes of the semiconductor under investigation with a comparable short test time and high radiation disposition.
The method described in the publication demonstrates a solution as to how the necessary cooling time, the so-called annealing, can be reduced. This time is necessary for the recombination of the induced holes following a high radiation disposition, to ultimately determine the influence of the radiation disposition on the reliability performance of the semiconductor according to the so-called MIL standard 883 TM 1019. In this connection, a temperature-dependent recombination process acting in opposition to the hole formation rate is described, whose formation rate likewise increases with increasing semiconductor temperature. The reason for this recombination is that the ions produced in the semiconductor oxide by radiation are unstable and therefore attempt to get back the missing electrons from their environment, whereby the original molecules or atoms are restored.